Soil acidification by intensified crop production in South Asia results in higher N2O/(N2 + N2O) product ratios of denitrification

Agricultural soils emit significant amounts of N2O to the atmosphere, and annual emissions are in some proportion to the input of reactive nitrogen to the system. Hence the ongoing intensification of cropping systems in South Asia will result in increased emissions of N2O. The prospects are potentially worse than those predicted by the increasing doses of N-fertilizers, however. The reason for this is that intensive cropping systems may acidify the soils, which could increase the N2O/(N2 + N2O) product ratio of denitrification due to interference with the expression of the different enzymatic steps in this process (Liu et al., 2010 FEMS Microbiol Ecol 72 407–417). We investigated this phenomenon for agricultural soils in the central mid-hills of Nepal. We sampled soils from fields that had been under intensified cultivation for ≥20 years, and adjacent fields with more traditional cultivation, in areas with permanently drained soils as well as areas with frequent flooding. The characteristic kinetics of NO, N2O and N2 production by denitrification in these soils was measured by anoxic incubations after flooding and drainage of the soils with 2 mM NO3−, to secure similar NO3-concentrations for all soils. The results demonstrate that intensification invariably lowered the soil pH and increased the N2O/(N2 + N2O) product ratios of denitrification. This effect of intensification was observed both for incubations with and without C-substrates (glutamic acid) added. The transient accumulation of NO varied grossly between sites, but was not affected by intensification. The results demonstrate convincingly that the intensification has resulted in higher intrinsic propensity of the soils to emit N2O to the atmosphere, and the correlation with pH suggests that acidification is responsible. This causal relationship is underpinned by emerging evidence that low pH interferes with the assembly of the enzyme N2O-reductase. We conclude that the ongoing intensification of agriculture in South Asia may result in severely increasing N2O emissions unless acidification of the soil is counteracted.